This invention relates to lasers and more particularly to a laser which can be advantageously used as a transmitter for a multipurpose optical radar, and for other purposes. The laser is particularly adapted for use in airborne radars of this type for use in low flying aircraft. In such an application, optical radars are often required to perform diverse functions such as, terrain following, terrain and obstacle avoidance, Doppler navigation, weapon delivery, radar imaging in two and three dimensions, moving target indication, etc.
No single modulation waveform can efficiently perform such a variety of functions. The present invention provides a laser which can be easily programmed to provide a large variety of modulation formats, which formats can be easily selected merely by changing the modulation waveform applied to the laser. Such a programmable modulated laser can form the transmitter of a multi-function airborne CO.sub.2 optical radar. Widely different modulation formats such as continuous wave (cw) at varying power levels, Q-switched pulses, RF modulated waveforms for both cw and Q-switched outputs, Q-switched and cavity dumped formats, as well as combinations of these, for example, shaped pulses comprising an initial high amplitude Q-switch pulse followed by a cw plateau of variable length. Many of these modulation techniques can be combined with mode locking to provide additional versatility.
A continuous wave optical radar transmitter can be used for example as a moving target indicator or for radial velocity measurement. Continuous wave outputs with a sinusoidal modulation are useful for example in three dimensional imaging. Terrain contouring radars require short high energy optical pulses such as can be obtained through Q-switching a laser. Certain experimental three dimensional techniques require Q-switch derived optical pulses with sinusoidal modulation thereon. Q-switching followed by cavity dumping can produce high energy pulses of variable width which are useful in terrain contouring. The aforementioned shaped pulses may be used in pulse Doppler applications wherein the intitial high amplitude pulse provides good radar range as well as range resolution and the lower amplitude trailing edge can be used as a frequency reference to measure target Doppler shift and hence radial velocity. The mode locked formats are useful mostly for experimental three dimensional radar imaging and for obtaining extremely short, high amplitude pulses.
In addition to its use as an optical radar transmitter, the novel laser is valuable as a laboratory signal source.
Various techniques are available for active modulation of CO.sub.2 lasers. These include acousto-optic modulation for extracavity frequency shifting and for intracavity mode locking; electo-optic modulation for mode locking, Q-switching and cavity dumping, chirp modulation; and Stark effect modulation for Q-switching as well as cavity dumping.
Intracavity electro-optic modulation offers the widest diversity of modulation formats for a carbon dioxide optical radar transmitter, considering such factors as efficiency, modulation rate, power handling capability, and modulation range. Specifically, an intracavity modulator which utilizes the voltage variable birefringence of a crystal to achieve polarization or coupling modulation offers the widest choice of laser modulated output formats. Such diversity is achieved by optimum design of the laser cavity and/or cavities and the use of a fully programmable voltage surce for application of a wide variety of modulating voltage waveforms to the birefringent crystal.
Intracavity electro-optic modulation of the type described is inherently more efficient and requires lower modulating voltages than extracavity modulators. Also, recent developments have made available high quality, low insertion loss cadmium telluride materials which make ideal modulator crystals for the 10 micron wavelength region. Also the development of a dual cavity laser with an electro-optical modulation system in an auxiliary coupled cavity wherein the power levels are much lower than in the adjacent main cavity has reduced problems associated with high power levels in electro-optic modulator crystals. This dual cavity laser is described and claimed in a co-pending application entitled, MODULATED INFRARED LASER WITH TWO COUPLED CAVITIES, Ser. No. 403,439, filed on July 30, 1982, U.S. Pat. No. 4,498,179.